1. Field of Invention
The present invention relates to a light scanning method and a light scanning device, and more particularly to a light scanning method in a light scanning device which deflects plural light beams emitted from a light source by reflection surfaces of a deflector, and simultaneously main-scans by the plural light beams deflected by the deflector a surface to be scanned, and to a light scanning device.
2. Description of the Related Art
Image recording apparatuses for recording images by light beams such as laser printers and electronic photocopiers are widely used. In such image recording apparatus, a photosensitive material is scanned by a light beam by means of a light scanning device.
In the light scanning device, generally, the light beam emitted from the semiconductor laser is modulated according to image data, and, via a collimator lens or the like, is incident on the reflection surfaces of a rotary polygonal mirror rotating at a predetermined speed.
By rotation of this rotary polygonal mirror, the light beam is deflected while its incident angle is being changed continuously, and the light beams scans the photosensitive material. The light beam reflected by the reflection surfaces of the rotary polygonal mirror is guided to the photosensitive material via an fxcex8 lens, a cylinder mirror (or cylinder lens), and the like, and scans the photosensitive material at a constant speed and is focused on the photosensitive material. As a result, an image is exposed and recorded on the photosensitive material.
Recently, to meet the need for higher speeds of image forming apparatus, improvements in the scanning speed of the light scanning device have been desired. As a technology for improving the speed of a light scanning device, the simultaneous scanning method has been known in which plural scanning lines are scanned simultaneously by one scanning by using plural light beams.
In designing the optical system of the light scanning device of this simultaneous scanning system (hereinafter called plural-beam scanning optical system), the bow difference and pitch deviation between the plural light beams must be considered. FIG. 9 shows bow difference occurring in the case of scanning with two light beams, and FIG. 10 shows pitch deviation when scanning with two light beams. In the diagrams, the broken line indicates an ideal scanning line position, and the solid line represents an actual scanning line position.
The bow difference means changes in the interval between two light beams in the subscanning direction depending on the scanning position in the main scanning direction, due to a difference in the curvature of the scanning lines formed by the light beams. The pitch deviation means widening or narrowing of the interval between scanning lines when two light beams are scanned simultaneously in a state in which the interval in the subscanning direction between the light beams deviates from a prescribed value. When scanning plural scanning lines simultaneously in one scanning by using plural light beams, if such bow difference or pitch deviation occurs, the image becomes uneven in the subscanning direction, and the image quality is lowered. Such bow difference and pitch deviation must be suppressed in order to obtain image output of high image quality.
Generally, bow difference and pitch deviation are more likely to occur the further the plural beams are apart from the optical axis of the optical system. Conventionally, the plural beams were usually only two or at most four beams which were arranged in a row. When the light emitting points outputting the respective light beams were arranged in a row at the light source, even if the interval between the actual light emission points was wide (see distance xe2x80x9caxe2x80x9d in FIG. 11), by inclining the entire light source obliquely in the subscanning direction, the interval between light emission points in the subscanning direction could be apparently narrowed (see distance xe2x80x9cbxe2x80x9d in FIG. 11), and deviation of the plural beams from the light source could be kept small. It was therefore relatively easy to decrease the bow difference and pitch deviation.
However, when the number of beams was further increased to further improve the speed, if the bow difference and pitch deviation were decreased by inclining the light source in the subscanning direction, on the contrary, the problem arose that the plural beams were widely apart in the mainscanning direction. Specifically, when the plural beams were apart in the main scanning direction, since the timing of each beam scanning a same point in the main scanning direction was different, the required buffer memory capacity for aligning the pixel positions in the main scanning direction increased, and a problem arose that the width of the reflection surfaces of the rotary polygonal mirror in the main scanning direction had to be increased substantially.
According to the technology disclosed in Japanese Patent Application Laid-Open (JP-A) No.5-294005, by disposing beams in a two-dimensional arrangement by using a VCSEL (Vertical Cavity Surface Emitting Laser Diode) as the light source, even if the number of beams is increased, the beams are prevented from greatly moving away from the optical axis of the optical system. However, in the case of the light source having light emission points disposed in a two-dimensional arrangement, unlike the conventional one-row arrangement light source, the interval between light emission points in the subscanning direction cannot be apparently narrowed by inclining the entire light source, and hence it is desired to solve the problems of bow difference and pitch deviation optically. So far, several methods have been proposed for reducing the bow difference or pitch deviation optically.
1) Using a f sin xcex8 lens: JP-A No. 1-163717
2) Reducing the subscanning direction lateral magnification rate of an optical system: JP-A No. 2-54211, JP-A No. 9-281421
3) Canceling mutual bow differences occurring at plural positions: JP-A No. 2-129614, JP-A No. 8-118725
4) Specifying intersecting positions of plural beams: JP-A No. 6-18802, JP-A No. 7-209596, JP-A No. 9-274152
5) Limiting tolerance of bow difference: JP-A No. 6-202019
6) Balancing subscanning direction image plane curvature and bow difference: JP-A No. 7-199109
7) Using an aspherical lens: JP-A No. 1-180510, JP-A No. 8-297256, JP-A No. 9-33850, JP-A No. 9-146030, JP-A No.10-333069, JP-A No. 11-84285
8) Aligning plural beams in the bow direction: JP-A No. 10-68898, JP-A No. 10-293260
1) Making plural beams incident, in parallel, onto the surface to be scanned: JP-A No. 7-209596, JP-A No. 9-274151
Thus, various technologies for reducing either one of bow difference and pitch deviation have been proposed, but in an actual light scanning device of a simultaneous scanning system, both bow difference and pitch deviation must be suppressed. Of the aforementioned technologies, only the technology disclosed in JP-A No. 7-209596 refers to both bow difference and pitch deviation.
The technology disclosed in JP-A No. 7-209596 is as shown in FIG. 12. Light beams 104, 106 emitted from laser diodes 100, 102 are incident on a rotary polygonal mirror A through a collimator lens 108 and a cylindrical lens 110. After the light beams 104, 106 incident on the rotary polygonal mirror A are reflected by the reflection surfaces of the rotary polygonal mirror A, they pass through an anamorphic motion compensation optical system (MCO) 112, 114 having an fxcex8 lens, a cylindrical lens, and a cylindrical mirror, and leave the light scanning device in a state parallel to the system axis (optical system optical axis) 116. That is, occurrence of bow difference and pitch deviation is suppressed by making the main exit beam of the light scanning device have a telecentric profile.
The cause of occurrence of bow difference in the plural-beam scanning optical system is, as mentioned above, that the light beam does not pass through the optical axis of the light beam. More specifically, the following two factors exist.
Light beams are incident on a rotary polygonal mirror 200 at an angle with respect to the subscanning direction (see FIG. 13).
Light beams pass outside of the optical axis of an fxcex8 lens 202 when passing through the fxcex8 lens 202 having power in the subscanning direction, or the beams enter the fxcex8 lens 202 at an angle with respect to the subscanning direction (see FIG. 14).
In the technology disclosed in JP-A No. 7-209596, the interval between the light emission points (interval between laser diodes 100, 102) of the semiconductor laser array used as the light source is as narrow as 25 xcexcm. Thus, there is no need to consider the angle in the subscanning direction on the rotary polygonal mirror A of the two light beams 104, 106 and the fxcex8 lens of the anamorphic motion compensation optical system 112, 114, and both bow difference and pitch deviation can be decreased.
However, as is clear from FIG. 12, since the two light beams 104, 106 are incident on the rotary polygonal mirror A at different angles in the subscanning direction, when the interval between light emission points in the subscanning direction is large, the incident angle in the direction of the light beam on the rotary polygonal mirror A is large, and a bow difference occurs due to deflection by the rotary polygonal mirror A. Further, since the light beams 104, 106 are incident at an angle outside of the optical axis of the fxcex8 lens (the anamorphic motion compensation optical system), if the interval between light emission points is large, similarly, a bow difference occurs.
The present invention has been devised to solve the above problems, and it is an object thereof to present a light scanning method and light scanning device capable of decreasing the bow difference and pitch deviation, regardless of the interval in the subscanning direction between light emission points for output of beams, when scanning simultaneously by using plural beams.
To achieve this object, a first aspect of the invention is a light scanning method comprising the steps of: (a) making plural light beams emitted from a light source incident at least onto reflection surfaces of a deflector in a mutually parallel state in a direction orthogonal to a main scanning direction; (b) deflecting the plural light beams by the deflector; and (c) focusing the plural light beams deflected by the deflector on a surface to be scanned, with an afocal relation between the reflection surfaces of the deflector and the surface to be scanned in the direction orthogonal to the main scanning direction.
A second aspect of the invention is a light scanning device which deflects plural light beams emitted from a light source by making the light beams incident on reflection surfaces of a deflector, and scans the surface to be scanned simultaneously by the plural light beams deflected by the deflector, the device comprising: (a) a first optical system for making the plural light beams incident at least onto the reflection surfaces of the deflector in a mutually parallel state in a direction orthogonal to a main scanning direction; and (b) a second optical system for focusing the plural light beams, which were deflected by the deflector, onto a surface to be scanned, with an afocal relation between the reflection surfaces of the deflector and the surface to be scanned in the direction orthogonal to the main scanning direction.
According to the first and second aspects of the invention, in the direction orthogonal to the main scanning direction (hereinafter called the subscanning direction), plural light beams are incident on the reflection surfaces of the deflector in a mutually parallel state (the light beams are not parallel light, but have the same advancing direction). That is, when the plural light beams are incident on the reflection surfaces of the deflector, their optical axes are mutually parallel in the subscanning direction, thereby preventing occurrence of bow difference due to deflection by the deflector.
Moreover, since the reflection surfaces of the deflector and the surface to be scanned are in an afocal relation (light beams are incident thereon and exit therefrom in a parallel state), the plural light beams deflected by the deflector are incident on the surface to be scanned in a parallel state. Thus, by setting such an afocal relation, for example, when the light beams pass through an fxcex8 lens, occurrence of bow difference from the reflection surfaces of the deflector until the light beams are incident on the surface to be scanned can be suppressed, and also occurrence of pitch deviation at times when there is fluctuation in the distance between the reflection surfaces of the deflector and the surface to be scanned can be suppressed.
That is, according to the first and second aspects of the invention, occurrence of bow difference and pitch deviation at the time of scanning the surface to be scanned simultaneously by plural light beams can be suppressed (or decreased).
In a third aspect of the invention, the light source emits the plural light beams in a mutually parallel state, and the first optical system may sets an afocal and conjugate relation between the light source and the reflection surfaces of the deflector.
In this case, the first optical system comprises a collimator lens for making the light beams emitted from the light source as divergent luminous flux into substantially parallel luminous flux, and a cylinder lens having power for condensing in the direction orthogonal to the main scanning direction, and focusing the light beams made into substantially parallel luminous flux by the collimator lens as a line which is long in the main scanning direction on the reflection surfaces of the deflector. The collimator lens and cylinder lens may be disposed such that the focal position at the light beam advancing direction downstream side of the collimator lens substantially coincides with the focal position at the light beam advancing direction upstream side of the cylinder lens.
For correcting the tilt of the reflection surfaces of the deflector simultaneously, the second optical system may focus the plural light beams deflected by the deflector on the surface to be scanned while setting a conjugate relation between the reflection surfaces of the deflector and the surface to be scanned.
In this case, the second optical system comprises an fxcex8 optical system having power for condensing only in the main scanning direction, a first cylinder optical system having power for condensing in the direction orthogonal to the main scanning direction, and a second cylinder optical system having power for condensing in the direction orthogonal to the main scanning direction. The first cylinder optical system and second cylinder optical system are disposed such that the focal position at the light beam advancing direction downstream side of the first cylinder optical system substantially coincides with the focal position at the light beam advancing direction upstream side of the second cylinder optical system.
The power of the first cylinder optical system for condensing in the direction orthogonal to the main scanning direction may be smaller than the power of the second cylinder optical system for condensing in the direction orthogonal to the main scanning direction.
The light source may be a VCSEL array having plural light emission points disposed in a two-dimensional arrangement.
Therefore, the present invention results in the excellent effect of decreasing the bow difference and pitch deviation, regardless of the subscanning direction interval of light emission points for output of beams, when scanning simultaneously by using plural beams.